Electric cartridge fuse having high operating temperature when carrying load current



F. J. KOZACKA ELECTRIC CARTRI May 19, 1970 3,513,424

DGE FUSE HAVING HIGH OPERATING TEMPERATURE WHEN CARRYING LOAD CURRENT Filed March 24, 1969 [III]!!! \{Ii'IIIII/I 'IIIII ZIIIIIIIIIIII I "6 m INVENTOR= FREDERICK J. KOZACKA M/WMA United States Patent Office ELECTRIC CARTRIDGE FUSE HAVING HIGH OPERATING TEMPERATURE WHEN CARRY- ING LOAD CURRENT Frederick J. Kozacka, South Hampton, N.H., assignor to The Chase-Shawmut Company, Newburyport, Mass. Filed Mar. 24, 1969, Ser. No. 809,631 Int. Cl. H01h 85/04, 85/10 US. Cl. 337158 4 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF INVENTION A major problem in the design of current-limiting fuses is reduction of the melting i -t value of a fuse, all other factors, such as current rating and voltage rating of the particular fuse, remaining unchanged. The melting i -t value, or thermal constant, depends upon the crosssectional area of the necks, or points of reduced crosssectional area, of the fuse link. The smaller that crosssectional area, the smaller the melting i 't value, or thermal constant, of the fuse of which the fuse link forms a component. Heat losses, or i -r losses, increase and decrease inversely to the cross-sectional area of the necks of a fuse link. To minimize melting i -t values the crosssectional area of necks must be minimized. To achieve an arc voltage suflicient to bring the let-through current from its peak rapidly down to zero calls for a relatively large number of serially related necks. If the cross-sectional area of necks is minimized, and the number of serially related necks large in relation to the voltage of the circuit in which the fuse is intended to be applied, this results in drastic increase of i 'r losses or heat losses. This increase in i -r losses, or heat generation, may be so large as to preclude the use of fuse casings of organic insulating materials, e.g. glass-cloth-melamine. Heretofore the use of casings of an organic insulating material was considered not to be feasible wherever the size of the casing was substantially smaller than prescribed by the NEMA standards, and the heat generation inside of the fuse when carrying its rated current was considerably above normal. In such instances, i.e. wherever it was a matter of size reduction, or miniaturization, coupled with the generation of relatively large amounts of heat and high casing temperatures under load or rated current conditions, one had to resort to casings of inorganic insulating materials as, for instance, steatite. It is, however, quite often undesirable to use casings of inorganic insulating materials because casings of certain organic insulating materials have desirable properties, e.g. freedom from cracking, or heat shock resistance, which cannot be duplicated with conventional inorganic casing materials.

Organic casing materials have a critical temperature which they can withstand for long periods of time without aging significantly. It is, therefore, desirable to evolve fuses having casings of organic insulating materials and fuse links which, when carrying their rated current, cause the casing to be heated to its aforementioned critical temperature. A smaller maximum operating temperature of the casing would indicate that the necks of the fuse 3,513,424 Patented May 19, 1970 links have not been reduced to the smallest cross-sectional area required to minimize the melting i -t, or the thermal constant, and/ or that the number of series necks is not sufficiently high to bring the let-through current from its peak as rapidly down to zero as possible. A larger maximum operating temperature of the casing would indicate that the necks of the fuse link have been reduced and/or their number increased to such an extent as to endanger the integrity of the casing material.

With these criteria in mind it is readily possible to determine the smallest permissible cross-sectional area of the necks of a ribbon fuse link of a fuse of given design, wherein the ribbon fuse link has a predetermined number of serially related necks. In such a fuse any significant overload, i.e. the flow of any current exceeding the rated current of the fuse, causes thermal damage to the casing if the overload lasts for a significant period of time. This then raises the problem underlying the present invention. The problem is how to protect the casing of the miniaturized fuse from thermal damage in cases of overload or, to be more specific, how to sever the current path through the fuse if and when the maximum temperature of the casing at its point of highest temperature exceeds a predetermined critical temperature.

One approach to this problem is to provide the fuse link with a link severing overlay of a metal having a lower fusing point than the base metal of the fuse link, which is silver. The overlay metal should preferably be tin, or an alloy of tin, because the link severing time of tin and its alloys is relatively rapid. With the conventional link-severing overlays of tin and alloys thereof on a fuse link one is, however, not capable of achieving the desired coordination between maximum permissible casing temperature and desired current ratings. These conventional link-severing overlays are generally intended to thermally protect an external load, e.g. the insulation of a motor, or transformer, etc., rather than the material of which the casing of the fuse is made. To constitute a link-severing overlay for the protection of a load such as a motor, a transformer, etc. results normally in such an arrangement, or configuration, of the link-severing overlay that the current path through the fuse is severed at a relatively high multiple of the rated current while the casing of the fuse is well below its above referred-to critical casing temperature.

It is possible by a novel and critical arrangement of a link-severing overlay of tin to correlate the above referred-to critical casing temperature and the rated current so that the casing will be substantially at its critical temperature when and as long as the fuse is carrying its rated current, and that the current path through the fuse will be severed if and when thermal damage to the organic casing of the fuse, or unduly accelerated aging thereof, is impending.

SUMMARY OF INVENTION Fuses embodying this invention include a tubular insulating casing which is of smaller size than that prescribed by the NEMA standards, and includes an organic material as a result of which it has a predetermined ceiling temperature, or maximum permissible temperature. The casing is closed by a pair of terminal elements. The casing has a partition which is relatively remote from one of said pair of terminal elements. There is a quartz filler inside said casing in the space bounded by said one of said pair of terminal elements and said partition. There is a non-fulgurite-forming arc-quenching filler in the space bounded by said partition and the other of said pair of terminal elements. A ribbon fuse link of silver projects through said partition and has ends conductively connected to said pair of terminal elements. Said fuse link defines a plurality of serially related necks including an axially outermost neck arranged in said non-fulguriteforming filler, which neck has an axially outer portion and an axially inner portion. A link-severing overlay of a metal including tin and having a much lower melting point than silver is arranged on said link adjacent said axially outermost neck at a point axially inwardly from said axially outer portion of said axially outermost neck.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a low-voltage fuse embodying this invention and is substantially a longitudinal section taken along I-I of FIG. 2;

FIG. 2 shows the structure of FIG. 1 and is substantially a longitudinal section taken along II-II of FIG. 1;

FIG. 3 is a transverse section taken along IIIIII of FIG. 1;

FIG. 4 is substantially a longitudinal section showing some parts in front elevation of a composite fuse including a plurality of fuse units as shown in FIGS. 1 to 3; and

FIG. 5 is a section along VV of FIG. 4.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings, and more particularly to FIGS. 1 to 3 thereof, reference numeral 1 has been applied to indicate a tubular casing including an organic insulating material. Casing 1 may be of glass-cloth-melamine and has a ceiling temperature, i.e., a temperature which it can withstand for long periods of time without significant deterioration, or aging. The ceiling temperature of any given casing material depends upon the test standards applied for its determination. Since these standards vary to some extent, somewhat different ceiling temperatures are often assigned to the very same organic insulating material. For this reason any published values in regard to ceiling temperatures should be considered as fair approximations rather than precise data. A reasonable ceiling temperature for the aforementioned glass-clothmelamine is 150 C., though some testing authorities assign a somewhat lower ceiling temperature to this material. Casing 1 is of miniaturized size, or of a smaller size than the size prescribed by the NEMA standards (see Low-Voltage Cartridge Fuses, National Electrical Manufacturers Association Pub. No. FU1-1963; table 35; or US. Pat. 3,413,585 to Frederick J. Kozacka for Electric Cartridge Fuse having Off-Center Fusible Elements, issued Nov. 26, 1968). A pair of terminal elements in the form of caps closes both ends of casing 1. The transverse partition 3 is arranged relatively remote from right terminal element 2 and relatively close to left terminal element 2, as seen in FIGS. 1 and 2. The relatively large space bounded by partition 3 and the right terminal element 2 is filled with a body of quartz sand 4 operating as an arc-quenching medium. The relatively small space bounded by partition 3 and the left-terminal element 2 is filled with a non-fulgurite-forming pulverulent arc-quenching filler 5, preferably gypsum powder. A ribbon fuse link 6 of silver projects through partition 3 which is provided to this end with a narrow slot. The axially outer ends of fuse link 6 project through like narrow slots in terminal caps 2 to the outer surfaces of the latter and are conductively connected to the latter by solder joints 7.

Fuse link 6 defines a plurality of points of reduced cross-sectional area generally referred to as necks. The plurality of necks 6a includes the axially outermost neck 6a arranged in the non-fulgurite-forming filler 5. Neck 6a has an axially outer portion adjacent to left terminal element 2, and an axially inner portion adjacent to partition 3. Reference character 8 has been applied to indicate a link-severing overlay of a metal including tin and having a much lower fusing point than that of silver. Overlay 8 is arranged immediately adjacent neck 6a at a point axially inwardly from the axially outer portion of neck 6a.

The above location of overlay 8 relative to neck 6a is critical inasmuch as the fuse does not properly operate unless the overlay 8 is positioned relative to neck 6a as above indicated. The neck 6a forms a temperature peak when the fuse is carrying current, and the temperature gradient on the axially outer side of fuse link 6 to the left of neck 6a, as seen in FIGS. 1 and 2, is much steeper than the temperature gradient on the axially inner side of fuse link 6 to the right of neck 6a, as seen in FIGS. 1 and 2. There are points to both side of neck 6a having exactly the same temperature, but the temperature difference within congruent areas to different sides of neck 6a is different. To be more specific, the maximum temperature difference prevailing within an area having a given geometrical configuration is larger when this area is situated to the right than when this area is situated to the left of neck 6a of link 6. Possibly this is the reason underlying the criticality of the location of overlay 8 relative to neck 6'.

The temperature of easing 1 is highest in the center region thereof. The temperature at this point depends upon the rate of heat generation in link 6 and the rate of heat transfer from link 6. The rate of heat generation in link 6 depends primarily upon the number of the necks defined by fuse link 6 and the cross-sectional area thereof. If casing 1 is of glass-cloth-melamine the number of the necks of fuse link 6 and the cross-sectional area thereof should be such as to result in a maximum temperature of casing 1 in the order of 150 C. when the fuse is carrying its rated current. In this context rated current means of the minimum fusing current.

When the rated current is exceeded to such an extent as ot seriously endanger the life of the casing 1 overlay 8 melts and effects by metal interdiffusion a relatively rapid interruption of the current path through fuse link 6. The point of break then formed is close to the left terminal cap 2, as shown in FIGS. 1 and 2. As a result, the burnback length available to generate the arc voltage required for interrupting the relatively small current then flowing through the fuse is relatively small. Small current arcs have the tendency to hang on, and require a large burnback length, or a substantial arc elongation, when the link-surrounding arc-quenching medium is quartz sand. The burnback length required for circuit interruption at small currents hardly exceeding the rated current of the fuse can be reduced to a minimum if a non-fulguriteforming arc-quenching medium, such as gypsum, is used to de-ionize the arc path. Quartz sand is a good conductor of heat and a good absorber of heat, and its use as arequenching medium in electric fuses is predicated on these physical characteristics. Non-fulgurite forming arcquenching media including gypsum are poorer conductors of heat and poorer absorbers of heat than quartz sand. As a result of the physical or thermal characteristics of nonfulgurite-forming arc-quenching filler 5, the temperature in the region of overlay 8 tends to be relatively high and the temperature gradient in that region tends to be relatively low. These are probably also important factors for achieving the intended safety thermostat action of linksevering overlay 8.

When it is necessary to achieve a relatively high currentcarrying capacity a plurality of current-limiting fuses as shown in FIGS. 1-3 may be integrated into a composite fuse as shown in FIGS. 4 and 5. As shown in these two figures casing 1a is closed by a pair of terminal plugs 9 whose axially inner end surfaces 9a are provided with recesses 9b. Each recess 9b in the left terminal plug 9 has a corresponding coaxial recess 9b in the right terminal plug 9. Each terminal plug is provided with six recesses. Terminal plugs 9 are held in position within casing 1a by steel pins 10, 11 projecting transversely through casing 1a into terminal plugs 9. Reference character F has been applied to indicate a plurality of fuses of which each is identical to the structure shown in FIGS. 13, and described in connection with these figures. The terminal caps 2 of fuses F are inserted into the recesses 9b in terminal plugs 9. Thus terminal plugs 9 are con ductively interconnected by six fuse units F. Each terminal plug 9 is provided with a blade contact 12 for insertion of the composite fuse into a fuse holder (not shown). Each blade contact has an oblong hole 12a through which a secrew-threaded rod may project for screwing blade contacts 12 against current-carrying con ductors as, for instance, bus bars.

Although this invention has been described in considerable detail, it is to be understood that such description of the invention is intended to be illustrative rather than limiting, as the invention may be variously embodied, and is to be interpreted as claimed.

I claim as my invention:

1. An electric current-limiting fuse, particularly for thyristor circuits, including in combination:

(a) a tubular casing including an organic insulating material having a predetermined ceiling temperature and being of smaller size than the size prescribed by the NEMA standards;

(b) a pair of terminal elements closing said casing;

(c) a transverse off-center partition inside said casing arranged more remotely from one of said pair of terminal elements than from the other of said pair of terminal elements;

(d) a quartz sand filler inside said casing in the space bounded by said one of said pair of terminal elements and said partition;

(e) a non-fulgurite-forming pulverulentarc-quenching filler in the space bounded by said partition and said other of said pair of terminal elements;

(f) a ribbon fuse link of silver projecting through said partition and having ends conductively connected to said pair of terminal elements, said fuse link defining a plurality of serially related necks including an axially outermost neck arranged in said non-fulguriteforming filler, said axially outermost neck having an axially outer portion and an axially inner portion; and

(g) a link-severing overlay of a metal including tin and having a much lower melting point than silver on said fuse link immediately adjacent to said axially outermost neck at a point axially inwardly from the said axially outer portion of said axially outermost neck.

2. An electric fuse as specified in claim 1 wherein said casing is of glass-cloth melamine, and wherein the number of necks of said ribbon fuse link and the crosssectional area of said necks are such as to result in a maximum temperature of said casing in the order of 150 C. when said fuse is carrying the rated current thereof.

3. A current-limiting fuse as specified in claim 1 Wherein said non-fulgurite-forming filler is gypsum.

4. A composite fuse including a plurality of currentlimiting fuses as specified in claim 1.

References Cited UNITED STATES PATENTS 2,599,646 6/1952 Kozacka 337-158 X 2,876,312 3/1959 Frederick 337-296 3,291,942 12/1966 Kozacka 337296 X HIRAM B. GILSON, Primary Examiner US. Cl. X.R. 337-296 

